1. Field of the Invention
The present invention relates to digital image and video processing. More specifically, the present invention relates to methods of converting interlaced video streams into progressive (i.e., non-interlaced) video streams.
2. Discussion of Related Art
Due to advancing semiconductor processing technology, integrated circuits (ICs) have greatly increased in functionality and complexity. With increasing processing and memory capabilities, many formerly analog tasks are being performed digitally. For example, images, audio and even full motion video can now be produced, distributed, and used in digital formats.
FIG. 1 is an illustrative diagram of a portion of interlaced digital video stream 100 most often used in television systems. Interlaced digital video stream 100 comprises a series of individual fields 100_1 to 100_N, of which the first ten fields are shown. Even fields contain even numbered rows while odd fields contain odd numbered rows. For example if a frame has 400 rows of 640 pixels, the even field would contains rows 2, 4, . . . 400 and the odd field would contains rows 1, 3, 5, . . . 399 of the frame. In general for an interlaced video stream each field is formed at a different time. For example, an interlaced video capture device (e.g. a video camera) captures and stores the odd scan lines of a scene at time T as field 100_1, then the video capture device stores the even scan lines of a scene at time T+1 as field 100_2. The process continues for each field. Two main interlaced video standards are used. The PAL (Phase Alternating Line) standard, which is used in Europe, displays 50 fields per seconds and the NTSC (National Television System Committee) standard, which is used in the United States, displays 60 fields per seconds.
Interlaced video systems were designed when bandwidth limitations precluded progressive (i.e., non-interlaced) video systems with adequate frame rates. Specifically, interlacing two 25 fps fields achieved an effective 50 frame per second frame rate because the phosphors used in television sets would remain “lit” while the second field is drawn. Progressive video streams use complete frames, including both the even and odd scan lines instead of fields. Because progressive scan provides better display quality, computer systems, which were developed much later than the original television systems, use progressive scan display systems. Furthermore, many modern televisions and television equipment are being developed to use progressive video streams. To maintain compatibility with existing interlaced video systems, modern progressive systems use deinterlacing techniques to convert interlaced video streams into progressive video streams.
FIGS. 2(a) and 2(b) illustrate a typical method of generating a progressive video stream 200 from an interlaced video stream 100. Specifically each field 100_X of interlaced video stream 100 is converted to a frame 200_X of progressive video stream 200. The conversion of a field to a frame is accomplished by generating the missing scan lines in each frame by copying or interpolating from the scan lines in the field. For example, as illustrated in FIG. 2(b) field 100_1 having odd scan lines 100_1_1, 100_1_3, 100_1_5, . . . 100_1_N, is converted into 200_1 by copying scan lines 100_1_X as odd scan lines 200_1_X, where X is an odd number and creating even scan lines 200_1_Y, where Y is an even number. Even scan lines 200_1_Y can be created by copying the preceding odd scan line 200_1_Y-1. This technique is commonly known as line repeat. Better results can be obtained using various interpolation schemes to generate the missing scan lines. For example, one interpolation scheme simply averages odd scan line 200_1_Y−1 with odd scan line 200_1_Y+1 to generate even scan line 200_1_Y. Other interpolation schemes may use weighted averages or other more complicated ways to combine data from the existing scan lines to generate the missing scan lines. Another normal mode deinterlacing technique known as 3D deinterlacing involves generating the missing scan lines by interpolating the missing pixels using data from adjacent fields. Deinterlacing by interpolation is not an integral part of the present invention. The principles of the present invention can easily be adapted to use any form of interpolation. However, many types of video streams are initially created as progressive video streams and then converted into interlaced video streams. For example, conventional motion pictures are captured and displayed using 24 frames per seconds. To display motion pictures on a PAL (interlaced 50 fields/second) display, each frame is separated into an odd field and an even field. Furthermore, PAL video devices are configured to slightly increase the field rate of the output interlaced video stream to achieve 50 fields/seconds. FIG. 3 illustrates this process. Specifically, FIG. 3 shows the first five frames M_1 to M_5 of a motion picture video stream MPVS being converted to 10 fields of an interlaced video stream 300. Field 300_1 includes the odd scan lines of frame M_1. Field 300_2 includes the even scan lines of frame M_1. Field 300_3 includes the odd scan lines of frame M_2 and Field 300_4 includes the even scan lines of frame M_2. In general a frame M_X is divided into an odd field 300_(X−1)*2+1 containing the odd lines of frame M_X and an even field 300_(X−1)*2+2 containing the even lines of frame M_X. For clarity, portions of interlaced video streams formed from an initial progressive video stream are said to be in “converted mode”.
While displaying interlaced video stream 300 on interlaced video systems provide adequate picture quality. Conventional deinterlacing techniques as described above and illustrated in FIGS. 2(a) and 2(b) can be used by a progressive scan display system to view interlaced video stream 300. However, the picture quality of a de-interlaced video stream formed from interlaced video stream 300 is much lower than the picture quality of the original progressive video stream that was used to create interlaced video stream 300.
Hence, there is a need for a deinterlacing method or system that can determine whether portions of an interlaced video stream are in normal mode (e.g. like a normal television signal) or in a converted mode (e.g. formed from an original progressive video stream). The method or system must then deinterlace the given interlaced video stream appropriately.